symbols: use proper Column number codepoint

Back when the colnr symbol was first introduced in
8929bc72a1 it included symbol `\ue0a3`
when powerline fonts where supposed to be used by setting
`:let g:airline_powerline_fonts=1`

However, it turns out, that those symbol, may actually not be defined in
the powerline fonts at all, only in the
[powerline-extra-symbols](https://github.com/ryanoasis/powerline-extra-symbols)
and this has caused various issues, because it either did not display at
all, or the symbol caused strange overflowing issues which made the
overall look of the status line not very appealing and already caused
various issues here in the vim-airline repository.

Therefore, fall back to the symbol `\u2105` (℅) which at least is
already defined in the Unicode specification and has therefore a higher
chance of being defined inside a powerline font at all (but it may still
be missing after all).

Also, it is more consistent, because it will now use the same symbol as
when `g:airline_powerline_fonts` is not defined and the default Unicode
symbols are selected.

So if you want to keep on using the old symbol, you need to use:
```
let g:airline_symbols.colnr="\ue0a3:"
```

fixes: #2563
related: #2381

autoload/airline/init.vim

@@ -122,7 +122,7 @@ function! airline#init#bootstrap()
     call s:check_defined('g:airline_left_alt_sep', "\ue0b1")  " 
     call s:check_defined('g:airline_right_sep', "\ue0b2")     " 
     call s:check_defined('g:airline_right_alt_sep', "\ue0b3") " 
-    " ro=, ws=☲, lnr=, mlnr=☰, colnr=, br=, nx=Ɇ, crypt=🔒, dirty=⚡
+    " ro=, ws=☲, lnr=, mlnr=☰, colnr=℅, br=, nx=Ɇ, crypt=🔒, dirty=⚡
     "  Note: For powerline, we add an extra space after maxlinenr symbol,
     "  because it is usually setup as a ligature in most powerline patched
     "  fonts. It can be over-ridden by configuring a custom maxlinenr
@@ -131,7 +131,7 @@ function! airline#init#bootstrap()
           \ 'whitespace': "\u2632",
           \ 'maxlinenr': "\u2630 ",
           \ 'linenr': " \ue0a1:",
-          \ 'colnr': " \ue0a3:",
+          \ 'colnr': " \u2105:",
           \ 'branch': "\ue0a0",
           \ 'notexists': "\u0246",
           \ 'dirty': "\u26a1",